Gaseous-discharge lamps, lamps in which light is generated when an electric current, or discharge, is passed through a gaseous medium, are not new to the lighting field. Fluorescent-type gaseous-discharge lamps have been employed for years to provide relatively efficient indoor lighting, such as for office buildings.
Unlike incandescent lamps, which are self limiting as a result of their positive-resistance characteristics, gaseous-discharge lamps have a negative-resistance characteristic. For this reason, gaseous-discharge lamps are operated in conjunction with a ballast, which provides the requisite current limiting. Traditionally, ballasts are of core and coil construction. One form is that of a simple choke, which provides an inductive impedance for current limiting. Another form is that of a transformer. The transformer form permits voltage conditioning, such as providing a high break-down potential, which is required for starting instant-start-type fluorescent lamps by ionizing to a plasma the gas therein. For rapid-start-type fluorescent lamps, the transformer includes a pair of windings for energizing the lamp filaments and, separating the pair of filament windings, a high-voltage winding having a high reactance for current limiting. Additionally, a magnetic shunt may be included in the transformer to limit the energy transferred through the magnetic path.
Unfortunately, traditional core-and-coil-type ballasts are relatively inefficient, having substantial heat generating losses that are generally equally divided between copper losses in the coil and core losses in the relatively inexpensive grades of iron employed therein. For example, it is not unusual for a traditional core-and-coil-type ballast employed in a dual 40 watt lamp fixture to dissipate from 15 to 20 watts, causing the ballast to run quite hot. Further, in many applications, such as in office buildings, this ballast-generated heat must be removed by air conditioning equipment, which is itself relatively inefficient. Another problem is that core-and-coil-type ballasts are relatively heavy, requiring that associated fixtures be more substantial than would otherwise be necessary.
The regulation afforded by traditional core-and-coil-type ballasts is, also, relatively poor. Typically, the operating level of fluorescent fixtures employing such ballasts varies as the square of the power-line voltage. Thus, in many applications, excessive lighting, dissipating excessive power, is often employed to insure that minimum lighting levels are achieved.
Among other problems associated with gaseous-discharge lamps is that they are less efficient when operated at the normal 60 Hz line frequency than when they are operated at higher frequencies. Fluorescent lamps are often difficult to start when cold and, as a result, flicker for some time. Fluorescent lamps require core-and-coil-ballast phasing both to reduce stroboscopic effects and to increase the power factor such lamps present to the AC power line via the ballast.
These problems are overcome by my "Electronic Ballast For Gaseous Discharge Lamps," which is disclosed in the U.S. Pat. No. 4,415,839. Briefly, the above mentioned ballast employs a power-factor-correcting network; a DC power supply; a pair of transistors (switches); a pulse generator; and a current-limiting network. To improve the power factor the DC power supply presents to an AC power line (by restricting the amount of power the DC power supply can obtain from the AC power line during peaks of the line cycle), the DC power supply is connected in series with the power-factor-correcting network across a 120 volt, 60 Hz, AC power line. The DC power supply is of a voltage-doubler type which develops on one line a twice-peak-potential level and which develops on another line a potential level one half the twice-peak-potential level, both with respect to a reference-potential level developed on yet another line. The transistors (switches) are connected in a totem-pole configuration in which the channels of the transistors are connected in series between the twice-peak-potential-level line and the reference-potential-level line. The pulse generator is configured to drive the gates of the transistors, in turn, so as to develop at a line at the juncture of the transistors, a source of high-frequency AC power, the waveform of which approximates a square wave. The current-limiting network is configured to couple one, or more, fluorescent lamps between the high-frequency AC power-source line and the return line. In one embodiment, the current-limiting network includes an inductor connected between the high-frequency AC power-source line and a node, a capacitor connected between the node and the return line, and another inductor connected in series with the lamp(s) between the node and the return line.
The above mentioned ballast is disadvantageous in that it provides little isolation from the AC power line. As a consequence, the above mentioned ballast may pose a safety hazard (danger of electrocution) to all who come in contact there with. Further, the above mentioned ballast is relatively complex and expensive.
The U.S. Pat. No. 4,333,139 of D. Owen et al discloses a "Static Inverter" which employs a transistor, a transformer, and a diode, all configured to drive a load. More specifically, the transistor is connected as a switch, with the source of the transistor connected to circuit ground and with the drain of the transistor coupled by a pair of serially connected windings of the transformer to a power supply potential. A pair of serially connected windings of the transformer are connected in series with the diode between the power supply potential and circuit ground. Another winding of the transformer is connected across the load. Although a load is shown in the D. Owen et al patent, it is indicated that the inverter is suitable for use in circuits for starting and re-starting gaseous discharge lamps. The transistor in the D. Owen et al patent is connected in a self-oscillatory configuration, and, thus, does not employ a separate oscillator. In addition, no network is employed in the D. Owen et al patent.
The U.S. Pat. No. 4,348,615 of R. Garrison et al discloses a "Discharge Lamp Operating Circuit" which employs the combination of an oscillator, a transistor, a transformer, and a network, all configured to drive a fluorescent lamp. More specifically, in the R. Garrison et al patent, the transistor is configured with the transistor base connected to the oscillator, with the transistor emitter connected to circuit ground, and with the transistor collector coupled by a transformer winding to a power supply potential. The R. Garrison et al patent discloses, as a "network" the series combination of a wi a capacitor and a lamp.
The reader may find of interest the U.S. Pat. No. 3,247,422 of H. Schultz; the U.S. Pat. No. 4,251,752 of J. Stolz; the U.S. Pat. No. 4,333,139 of D. Owen et al; the U.S. Pat. No. 4,348,615 of R. Garrison et al; the U.S. Pat. No. 4,415,959 of P. Vinciarelli; the U.S. Pat. No. 4,613,796 of D. Bay; and the U.S. Pat. No. 4,802,078 of J. Hill.